The invention relates to a suppressor which also acts as an insulating member for voltage-conducting parts having user-accessible metal portions. The suppressor-insulator member is especially suitable for electric power tools and apparatus which are double-insulated and have housings which are made in part of metal.
Double insulation is a term used to characterize electric motor driven devices that have been equipped with supplementary insulation as well as functional insulation. Functional insulation denotes the insulation necessary for the proper functioning of the tool and for basic protection against electric shock as for example the conventional winding insulation of the pole pieces; whereas, supplementary insulation denotes an independent insulation provided in addition to the functional insulation, in order to ensure protection against electric shock in the event of failure of the functional insulation. Thus, in a double-insulated tool, if the functional insulation breaks down or is by-passed for some reason, a protective barrier remains to prevent injury to the user. For example, an electric tool can have an electric motor whose armature shaft is normally connected directly to an output element by means of which the tool produces useful work. In addition to the insulation on the wires which are on the armature, the double insulated tool would have a second barrier of insulation between the armature lamination stack and the tool element of the device. The second barrier of insulation provides the supplemental protection against possible injury to the user of the tool and can for example be provided by means of a sleeve positioned on the armature shaft. In other cases, the insulated gears can be provided by building insulating members into the armature shaft. There are of course still other arrangements which can be utilized to provide double insulation.
Further information relating to the insulation terms described herein may be had by making reference to the handbook entitled: "Specification for Hand-Held Motor-Operated Tools" published for the International Commission on Rules for the Approval of Electrical Equipment by Nederlands Comite voor de CEE, Utrechtseweg 310, Arnhem, Netherlands.
Double insulated tools like other tools generate high-frequency noise. This high-frequency noise generated in a power tool has generally two (2) components, namely: a symmetrical component generated internally such as from the brush action; and, a asymmetrical component also generated within the tool by brush action and becomes a problem as a consequence of the impedance to ground of a person holding the tool.
Electric apparatus such as electric power tools driven by electric motors such as drills, circular saws, and the like must be provided with noise suppression means and for this purpose generally capacitors and/or chokes are utilized for short-circuiting or conducting away the high-frequency noise. It is also known to combine capacitors and coils to form a filter which can affect the symmetrical noise component.
However, it is still more difficult to conduct away noise caused by inductive or capacitive action in the machine and wherein the noise appears as asymmetrical. For this purpose, it is already known to use so-called x-y capacitors; that is, a circuit configuration of three capacitors of which the x capacitor is connected between the lines of the source voltage; whereas, the two y capacitors have respective terminals connected to corresponding ones of the lines and with the other respective terminals of these y capacitors mutually connected at a common terminal. This common terminal is then, in turn, connected to a mass such as the laminated stator stack of the field if this stack is electrically insulated with respect to metal parts touchable by the operator of the tool.
It is, however, not permitted to use such an x-y capacitor for short-circuiting noise on operator-accessible metal parts such as the gear housing of a double-insulated drill, since the danger exists that a capacitor can be defective and lead to a voltage break-down from the voltage conducting parts to the operator-accessible metal parts. In order to eliminate this danger, German and international safety requirements call for providing a minimal isolation between the voltage-conducting parts and the operator-accessible metal parts. According to these requirements a definite isolating spacing is required for maintaining a voltage break-down gap of a predetermined length as well as a minimal creep length. Furthermore, the conditions for the leakage current and the resistance to high-voltage must be maintained.
Accordingly, up until now, it was impossible to bleed away high-frequency noise on operator-accessible metal parts or to short circuit the same in a simple manner without clashing with the safety standards.